This invention relates to a microporous film of high molecular weight polyolefin useful in the preparation of filter mediums or non-aqueous battery separators and to a process for producing the film. More particularly, it relates to a microporous film of high molecular weight polyolefin, which contains vein-like fibrils as a main constituent on each of which fibrils crystallites flocculate and to a process for producing the microporous film of high molecular weight polyolefin, which comprises subjecting a gas-impermeable film to a heat treatment and, if necessary, to a stretch treatment to thereby render the film microporous.
Many processes for producing microporous films of high molecular weight polyolefins have so far been proposed as can be seen in, for example, Japanese Examined Patent Publication Nos. 6-53826, 6-2841 and 7-17782.
According to these processes, microporous films are produced by adding a plasticizer composed of a low molecular weight compound such as a hydrocarbon solvent (e.g., decane, dodecane, decalin, paraffin oil or mineral oil), a fatty acid or its aliphatic hydrocarbon derivative (e.g., a fatty acid ester or an aliphatic alcohol), a paraffin wax, or dioctyl phthalate or dibutyl sebacate to a high molecular weight polyolefin forming the mixture into a film, then removing said low molecular weight compound from the film.
Further, according to the processes proposed by Japanese Examined Patent Publication Nos. 6-53826 and 6-2841, the film from which the low molecular compound has been removed is stretched.
However, the microporous films produced by Japanese Examined Patent Publication Nos. 6-53826 and 6-2841 undergo closing of the micropores only when heated up to a temperature of about the melting point of the film (poor closing properties) or, in some cases, the micropores do not disappear at a temperature lower than the melting point.
As a process for improving the closing properties, there have been proposed, for example, a microporous film produced by using a composition having a wide molecular weight distribution prepared by adding, to an ultra-high molecular weight polyethylene, a polyethylene having a molecular weight lower than that of the ultra-high molecular weight polyethylene (Japanese Unexamined Patent Publication No. 3-105851) and a process of melt stretching (draft) a composition of an ultra-high molecular weight polyethylene and a plasticizer into a film.
The microporous film disclosed by Japanese Examined Patent Publication No. 7-17782, however, has a poor strength in comprarison with other microporous films obtained by the aforesaid processes due to not having been stretched, though it undergoes closing of micropores when heated to a temperature lower than its melting point. In addition, the microporous film obtained by the process disclosed in Japanese Unexamined Patent Publication No. 3-105851 has a similarly poor strength due to the presence of a large quantity of low molecular weight polyethylene and, in some uses, might be broken.
On the other hand, as a process for producing a microporous film without adding any low molecular weight compound, there have been proposed processes described in Japanese Examined Patent Publication Nos. 6-18915 and 2-19141. These processes include stretching of film to render it microporous as a necessary step, and use a polyethylene having a molecular weight as low as 0.2 to 20 g/10 min in terms of MFR, thus providing microporous films having a tensile strength of up to 0.02 GPa or, at the highest, 0.03 GPa which limits the industrial use thereof.
As a result of various investigations to obtain a microporous film having a high strength without adding any low molecular weight compound which serves to render the film microporous but requires a subsequent step of removing itself from the formed film, the inventors have found that a microporous film obtained by subjecting a specific high molecular weight polyolefin film to a heat treatment and, if necessary, a stretch treatment posesses enough high strength and closing properties, thus having achieved the present invention.